Boston — Scores of devices to help prevent motor vehicle thefts are on the market.
Few, however, have proved completely effective, according to those close to the scene.
''About the best that can be done is make it so hard to break in that the would-be thief gets discouraged,'' suggest auto insurance industry officials, many of whose firms provide special premium deductions for motorists whose vehicles are equipped with antitheft equipment.
Carmakers, too, are ''doing what they can'' to help prevent their autos and trucks from being stolen. But such gains are usually short-lived, laments Thomas J. Carr, of the Motor Vehicle Manufacturers Association of America.
He notes that a few years ago General Motors Corporation thought they had the challenge at least partially met when they came up with separate keys to the doors and the ignition. ''Car thefts declined for about six months, then they began to increase as thiefs figured another way to steal cars,'' he recalls.
Automakers, he explains, are ''trying to keep track of the different ways vehicles are stolen in hopes of keeping ahead of the car thieves.''
Alarm systems, as good as they may be, often are of little value when the car is parked in a lot or on the street in a poorly lighted area, Mr. Carr emphasizes, adding that such devices are more apt to fend off would-be burglars if the car is in the owner's yard or in front of his home.
The best protection against having a vehicle stolen is to remove the keys from the ignition, lock the car, and keep it in a well-lighted, easily visible location when not occupied, he says.